This is, in my opinion, completely unacceptable.
If it were a vampire PC, to emote mists taking shape and moving towards people, interacting in any way...they would need to show themselves in mist form to do so, considering other players were seeing tendrils of mists encircle one PC, and a misty form stalk over to another PC and antagonize them. This did -not- happen, as I was there. There were only yellow messages to cover it, with basically no visuals whatsoever.
If you're in hiding, using HIPS or otherwise...to interact with ANYONE physically, you must come out of hiding to do so. If this is who I think it is, then this is the second time HIPS has been abused.
Monstrous PCs should not be greiving like this in any way, shape or form (and 90% of them don't. Cheers to them!), because they applied for the template with specific purpose of enhancing the setting, and not causing OOC frustration.
If it was any other monstrous PC...well, look at one of the rules of the server. Whoever it was did not have control over mists because it is in no way on their character stat sheet, was not upholding any RP, and also not within the limits of the server engine.
Wights, Ghouls, Mummies, Werecreatures...basically any template other than a vampire cannot have access to "Mist" abilities as were used in this fashion. A very powerful vampire could likely physically shape their mist form to create tendrils, or shapes to promote fear, but they would have to SHOW themselves to accomplish this...
Hence the term CHEESING.
This is again, in my opinion, completely unacceptable, and should be looked into by the DM Team.